Lunacy and astronomy

In praise of moons

They’re number two. They try harder

TO BE parochial on an astronomical scale sounds like a tall order. Yet people manage it all the time. When they think about the possibility of life in other parts of the universe, they think of it happening on planets. But this is simply local prejudice. Moons, which Earthlings tend to see merely as adornments to the night sky, are just as good a place to look for life as planets are, perhaps even better. Which is why astronomers are trying to spot the moons of planets around other stars (see article). And life is just a part of it. To anyone not hung up on hierarchies of who-orbits-whom, moons are in all manner of ways more various and more intriguing than planets.

It is true that moons are, on average, small. But big moons can be larger than small planets. And what they lack in size moons make up for in variety and activity. Their surfaces range from the stark cratering of Earth’s moon through the cantaloupe-skin of Triton, which circles Neptune, to the cue-ball-smooth ice of Europa and the lurid leprous sulphur of Io, both companions of Jupiter. And though the three rocky planets that are not the Earth—Mercury, Venus and Mars—appear to be geologically quiescent, that is not true of many moons. Spacecraft passing both Triton and Enceladus, a satellite of Saturn, have seen geysers that dwarf those of Iceland or Yellowstone, and the volcanoes of Io are in constant tumult.

Planets do have an edge when it comes to atmospheres. Venus, Earth and Mars all boast thick enough envelopes of gas to have weather; among the moons only Saturn’s Titan has anything to speak of in that department. But in the matter of oceans—widely seen as a near-necessity for life—the moons do well. Venus and Mars long ago misplaced any seas they may once have had, leaving Earth the only oceanic planet. Earth’s oceans, though, are smaller than those beneath the icy crust of Europa—and Europa’s siblings Callisto and Ganymede may well have subsurface oceans, too, as may Triton. Titan, meanwhile, has rivers, clouds, rains and lakes of methane.

In other star systems, moons have many advantages as abodes for life. One of the discoveries that has come with the ability to detect planets around other stars is that planetary systems are knockabout places in their early years. A lot of small planets get flung out of their orbits into outer darkness. This is less likely to happen to such a body if it is the moon of a large planet, so moons may survive preferentially. And moons can be hospitable where planets are not. An Earth-sized planet too close to its star will end up with one face permanently illuminated and one eternally dark. But a moon going around a large planet in such an orbit is saved from such a fate; as it orbits its parent orbiting the star all of its surface will, from time to time, be bathed in the star’s welcome light.

In the heavens as it is not on Earth

Moons orbiting planets far away from their stars may also be more hospitable to life than Earth-sized planets would be in similar orbits. A big planet gives off heat. That warms its moons. And further warmth is available from tides. The tides Earth’s moon raises on its parent can be powerful things. Those raised by planets on their moons are more powerful still, since a planet is bigger, and thus has stronger gravity, than its moons. It is the heat produced by constant tidal kneading of their interiors that makes Enceladus spout geysers and Io volcanoes when planets of their size would sit inert.

No one can say whether life is more likely on moons than on planets—but astronomy stands ready to answer such questions over the coming decades. Humans assume that the way things are on Earth is the way things must be. Science is there to help them widen those narrow minds.